1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method, and a program.
2. Description of the Related Art
Conventionally, it is well-known that on an image formed by an image forming apparatus, such as a color printer or a color multifunction peripheral (MFP), periodic uneven density occurs in a sub-scanning direction on the page in accordance with the circumferential length of a photosensitive drum. To suppress the effect of such uneven density in the sub-scanning direction, there is already known a technology to output a patch image in which respective same tone patches in cyan (C), magenta (M), yellow (Y), and black (K) colors are staggered by a distance corresponding to half the length of a period of uneven density in two positions in the sub-scanning direction at the time of calibration, and to create a tone correction table on the basis of an average value of colorimetric values of each color, thereby offsetting the uneven density and setting a tone correction table is unaffected by the effect of uneven density.
For example, in a technology disclosed in Japanese Patent Application Laid-open No. 2007-264364, there is achieved a method of adjusting the density by patch formation taking into account periodic uneven density caused by a fluctuation in the rotating speed of an image carrier and the like. Specifically, Japanese Patent Application Laid-open No. 2007-264364 discloses an image forming apparatus which generates patches with the same color and the same density and tone value in two positions kept at a predetermined distance (a distance corresponding to half the length of the period of uneven density or the like) in a circumferential direction of an image carrier, measures the density of each of the patches generated in the two positions, and generates information for a density correction process on the basis of the measured densities (for example, an average value of two densities).
However, the conventional technology to stagger patches with the same tone by a distance corresponding to half the length of the period of uneven density in the two positions in the sub-scanning direction is intended for single-color calibration, and is not particularly considered for mixed-color calibration.
Even when a calibration process is performed so that primary colors (C, M, Y, K) meet respective target values, a secondary color and a tertiary color may not be stably output in the same color; so, the necessity for mixed-color calibration is required. Namely, because most of colors contained in a natural image are mixed colors, an image forming apparatus is required to perform mixed-color calibration so as to stably output mixed color in the same color constantly.
In the mixed-color calibration, a single-color patch formed of a single color material only as well as a mixed-color patch formed by overlaying a plurality of color materials are output, and a tone correction table is created on the basis of colorimetric values of the single-color patch and the mixed-color patch.
However, in a conventional mixed-color calibration, to suppress the effect of uneven density in the sub-scanning direction on the page, all patches need to be staggered by a distance corresponding to half the length of the period of uneven density in two positions. Therefore, the mixed-color calibration causes an increase in the number of patches, resulting in an increase in the number of pages for a patch image, i.e., an increase in the number of recording media, such as sheets, required to output the patch image.